A field trial of the rhesus rotavirus (RRV) vaccine was begun in Umea, Sweden in January 1985 to examine the efficacy of the vaccine given at a dose of 1 million pfu. 106 infants 4-12 months of age were given placebo or vaccine with a bicarbonate-citrate buffer at the start of the rotavirus season. These individuals will be followed actively for rotavirus diarrhea and serologic responses for 18 months. In the first month of followup, vaccinees had a significantly greater number of low grade fevers and loose stools than placebo recipients. Consequently, 2 phase I trials are being planned before recruitment of more infants into the trial continues. In one trial, reactogenicity of a lower dose of vaccine and the importance of the buffer will be examined by giving groups of 15-20 infants aged 4-12 months a lower dose of the vaccine with the bicarbonate-citrate buffer and without buffer. A third group will receive placebo alone. In the second phase I trial, immunogenicity and reactogenicity of the lower dose vaccine will be examined among infants 1 month of age. In the field trial, about 40 infants have had diarrhea during the first rotavirus season so this trial could potentially be the first to establish efficacy of the RRV vaccine.